Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors has continued to advance at a great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these devices.
Image sensors conventionally receive light on an array of pixels, which generates charge in the pixels. The intensity of the light may influence the amount of charge generated in each pixel, with higher intensity generating higher amounts of charge. The charge may be converted into a digital representation of the charge by analog-to-digital converter (ADC) circuits in the image sensor based on a comparison to a reference voltage signal. However, if the amount of generated charge is more than the voltage range of the ADC circuits, the digital representation of the charge may not be correct.
Many techniques have been employed to increase the voltage range of the ADC circuits. However, some of these techniques may not provide the desired range.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.